It has long been desirable to achieve longer wearing non-stick resin coatings on metal substrates. U.S. Pat. No 5,411,771 (Tsai) discloses electric arc spraying at 100-300 amps current and air pressure of 5-8 kg/cm.sup.2 metallic materials including copper, zinc, nickel, chromium, aluminum, carbon steel and other stainless steels onto a metal substrate which is made of iron, steel, copper or aluminum to form a mechanically resistant layer on the substrate, with the result that the 23-36 micrometer thick polytetrafluoroethylene (PTFE) non-stick layer applied to the mechanically resistant layer has a pencil hardness of 8-9 H, which is disclosed to make the non-stick layer abrasion resistant. The patent discloses the preference for a stainless steel substrate and low carbon stainless steel as the metal wire for arc spraying to form the mechanically resistant layer. U.S. Pat. No. 5,462,769 (Tsai) improves upon the '771 patent by forming the non-stick coating of PTFE/perfluoroalkoxy polymer (PFA) to eliminate rusting of the cooking surface. Both patents require the metal substrate, prior to the metal spraying step which formed the mechanically resistant layer, to be roughened, and in particular, use aluminum oxide particles blasted against the substrate surface to create a surface roughness of Ra 4.5-5.5 micrometers (177-217 microinches). The mechanically resistant layer formed on this roughened surface has a generally greater roughness of 5-8 micrometers (197-315 microinches), leading to the non-stick layer having a roughness of 2.5-5.5 micrometers (98-217 microinches).
One disadvantage of the process of these patents is the requirement for blasting the substrate surface to roughen prior to metal spray coating. Such blasting is a difficult, costly and environmentally unfriendly process in a manufacturing operation, especially cookware manufacturing, because of the:
high consumption of compressed air; PA1 effect of abrasive airborne dust on rotating machine parts in the surrounding areas, leading to increased maintenance costs; PA1 elevated noise levels in the area surrounding the grit blasting operation, requiring hearing protection for operating personnel; and PA1 potential process bottleneck resulting from grit blasting machine maintenance requiring downstream shutdown.
In U.S. Pat. No. 5,069,937 (Wall) discloses that aluminum is not a good substrate for spraying with molten metal because of the formation of aluminum oxide within the coating layer, which becomes noticeable as "white rust" corrosion. Wall claims to have solved this problem by using a particular stainless steel as the molten metal, namely that which contains 25 to 35 wt % chromium, 8 to 15 wt % nickel, with most of the remainder being iron. Wall also roughens his substrate before spraying with the molten metal, the roughening forming peaks and valleys, with the distance from peak to valley being 15 to 20 micrometers (591-787 microinches). This extreme roughening is made even rougher by the molten metal droplets forming particles on the substrate surface of 25 to 50 micrometers (984-1969 microinches) in diameter.
Beside having the disadvantage of requiring roughening of the substrate, this teaching has additional disadvantages. The depth of roughening (valleys) is not reliably, on a production basis, completely fillable with fluorocarbon polymer layer, even when applied as a liquid dispersion, leaving small air pockets between the depth of the valleys and the underside of the polymer non-stick coating. Eventually, this air and other gas permeating through the coating expands under heating to cause the coating to separate from the sprayed coating on the substrate, representing a failure of the non-stick coating. Another disadvantage arises from the application of stainless steel of varying nickel and chromium compositions on aluminum causing corrosion of the metal substrate. This corrosion arises when such substrates are exposed to electrolytic environments, such as tomato sauces, commercial powder dish washing detergents and the like, the metallurgical phenomenon of galvanic (bimetallic) corrosion causes accelerated corrosion of the aluminum substrate. This corrosion causes blistering of the nonstick coating, leading to loss of release and failure. Such corrosion has been observed even with stainless steel alloys of high chromium and increased nickel content as described by Wall.